U.S. Pat. No. 6,346,805 B1 describes an apparatus for current measurements in a wide range in which a first signal generated in a low-power measurement transformer with a magnetic core and a second signal generated in a nonmagnetic measurement transformer like a Rogowski coil are compared. Depending on the degree of accuracy, one of the two signals is then selected by comparing the signals from both measurement transformers and evaluating the deviation between the signals as a criterion for selecting a signal and then the selected signal is further processed. The low-power measurement transformer is used in the actual metering range and in large parts of the protective range, whereas the Rogowski coil is merely used in the protection range to detect high, stationary or displaced currents. A Zener diode is used as a protection element in order to limit the voltage generated in the Rogowski coil and to guarantee electromagnetic compability. The apparatus in U.S. Pat. No. 6,346,805 B1 needs two measurement transformers for the protection and metering range. As it has to be estimated for each signal which signal is selected according to the required accuracy, a fast signal processing system is needed. The apparatus can only be used in a predetermined metering range depending on the properties of the low-power measurement transformer and a predetermined protection range depending on the properties of the Rogowski coil and the upper limit of the A/D converter. It is not intended to use the apparatus in other metering and/or protection current ranges.
De 42 04 515 describes a method to measure metering and protection currents with a Rogowski coil. By a switch the signals are transmitted to an A/D converter either as amplified signals by a gain element or as unamplified signals. A calculation unit which is connected to the AD converter controls the switch and the gain element. The gain factor by which the signals are amplified is chosen by first applying the highest possible gain factor. In case the amplified and digitized signal exceeds the range of the A/D converter a lower gain factor is chosen until the digitized value is below the range of the AD converter. The lowest possible value is the unamplified signal from the Rogowski coil.
In IEEE PES transmission and distribution conference, p. 827-830 by Sohn J M et al. a system is described for measuring metering currents with a Rogowski coil. A resistor block is connected to the Rogowski coil in order to adjust the rated current range.
DE 34 25 390 describes a system for the measurement of a current with a magnetic core and a coil, which system can be operated in different measurement ranges by electrically connecting a different number of windings of the coil to an electronic device.